Electron fishbones: Theory and experimental evidence

TL;DR

This paper explores the excitation mechanisms of electron fishbone instabilities driven by supra-thermal electrons, analyzing experimental and theoretical aspects relevant to plasma stability and fusion reactors.

Contribution

It provides a comprehensive theory and experimental evidence for electron fishbone instabilities driven by ECRH and LH power, including mode stability and transition dynamics.

Findings

ECRH and LH can independently excite electron fishbones.

Transition from steady oscillations to bursting is influenced by LH power.

Results are relevant for understanding alpha particle interactions in fusion plasmas.

Abstract

We discuss the processes underlying the excitation of fishbone-like internal kink instabilities driven by supra-thermal electrons generated experimentally by different means: Electron Cyclotron Resonance Heating (ECRH) and by Lower Hybrid (LH) power injection. The peculiarity and interest of exciting these electron fishbones by ECRH only or by LH only is also analyzed. Not only the mode stability is explained, but also the transition between steady state nonlinear oscillations to bursting (almost regular) pulsations, as observed in FTU, is interpreted in terms of the LH power input. These results are directly relevant to the investigation of trapped alpha particle interactions with low-frequency MHD modes in burning plasmas: in fact, alpha particles in reactor relevant conditions are characterized by small dimensionless orbits, similarly to electrons; the trapped particle bounce averaged dynamics, meanwhile, depends on energy and not mass.